The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resin compositions are typically used as the encapsulating resin because they generally have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Nowadays, semiconductor devices have an increasing degree of integration and the chip size is accordingly increasing. On the other hand, packages of smaller outer size are desired in order to comply with the size and weight reduction of electronic equipment. With regard to the attachment of semiconductor parts to circuit boards, the surface mounting of semiconductor parts is widely employed since the surface mounting can accommodate demands for a higher density of parts on boards.
When semiconductor devices are surface mounted, however, it is commonly employed to immerse the semi-conductor devices entirely in a solder bath or pass the semiconductor devices through a hot zone where solder is melted. Due to the thermal shocks encountered in such steps, the encapsulating resin layer will crack and the encapsulating resin can separate from the lead frame or chip at their interface. Such cracks and separation become more outstanding if the encapsulating resin layer on the semiconductor device has absorbed moisture prior to the thermal shocks upon surface mounting. In practical working steps, the moisture absorption of the encapsulating resin layer is inevitable. Then the epoxy resin-encapsulated semiconductor devices can lose reliability at the end of surface mounting. Several attempts were made in the prior art to insure reliability, for example, by increasing the filler loading to reduce moisture absorption for avoiding the risk of popcorning, by reducing the viscosity of the epoxy resin composition for facilitating the formation of thin packages, and by using fast curing catalysts for improving productivity and molding cycles.
However, prior art curing catalysts such as tertiary amine compounds, tertiary phosphine compounds and derivatives thereof give rise to the problem of a viscosity increase during mixing.
Imidazole derivatives are also known as a curing accelerator for epoxy resin compositions. JP-A 76420/1983, 103525/1983 and 100128/1982 and JP-B 18853/1994 disclose that an epoxy resin composition having good shelf stability and curing into a product having improved adhesion is obtained using curing agents obtained by reacting 2-methylimidazole with pyromellitic anhydride and trimellitic anhydride.
These curing catalysts, however, have poor electrical properties and tend to show an increased percentage of rejects in a reliability test. Simply blending such a curing agent with a curing accelerator is difficult to obtain an epoxy resin composition which is shelf stable and smoothly flowing and cures into a product having moisture resistance and adhesion.